ARCHIVED: What is meant by the speed of a CD or DVD drive?

CD and DVD drives

The speed of a CD or DVD drive is normally stated in terms of multiples of the original speed of the original standard. Terms like "double speed" or "24x" refer to multiples of that original speed. Originally, CD-ROM drives had transfer rates of only 150KB/second, which was slow compared to much higher rates for the then current IDE hard drives. Later developments in CD technology allowed the development of CD-ROM players that spun the CDs several times as fast; those models transfered at a 2x speed, (300KB/second), then later at 4x (600KB/second), and so on until today, where the newest CD-ROM drives can transfer up to 72x the original speed (10,800KB/second). Following the naming convention, those drives are considered 72x CD drives.

CD-ROM average seek times range from 200 milliseconds (ms) to as low as 80ms. In comparison, current hard drive seek times are typically around 9ms, and some are lower.

The fastest drives use Constant Angular Velocity (CAV) technology. CD-ROM drives usually operate using Constant Linear Velocity (CLV). Since the circumference at the edge of the CD-ROM is longer than at the center, and because the data on a CD-ROM are written at the same linear density everywhere on the disk, the CD-ROM spins faster when the tracks toward the center are being read. The new, faster CD-ROM drives, using CAV, will read data at the outer edge of the CD-ROM faster than data near the center. This accounts for the variable data transfer rates that one finds in advertisements and specification sheets for these drives.

Some older drives use Zoned CAV (ZCAV) or Partial CAV (PCAV). The drives will increase the spin rate as the head moves towards the middle of the disk, but it is done in discrete steps rather than continuously.

Note: There are no tracks on a CD-ROM; all of the data is contained in one unbroken spiral.

Nowadays, with drives that can read, write, and rewrite (i.e., erase an RW disc and then write over it), you'll see drives with multiple speed ratings, for example, 52x/24x/16x. That normally refers to the speed at which a drive can read, write, and rewrite in that order.

DVD drives

Nearly everything above applies to DVD drives too, with just a couple of exceptions:

1. The original speed rating for DVDs is different. For a DVD, 1x is 1352.54KB/second (that's a bit over the 9x CD drive speeds). Following the convention, 2x for a DVD drive is 2,705KB/second, 4x is 5,410KB/second, 8x is 10,820KB/second, and 16x would be 21,640KB/second.
2. When a DVD+/-RW lists its multiple speeds, it is not in the order of read, write, and rewrite. A 16x/4x/16x does not read at 16x, write at 4x, and rewrite at 16x (rewrites are slower than simple writes). It's best to read the documentation for the drive and see what individual function speeds are to judge the overall speed of the drive.